1. Field of the Invention
The present invention relates to the manufacture of a solid state electrochemical cell, and more particularly, a method for producing a rechargeable solid state laminar cell including a lithium anode, an electrolyte, a composite cathode and a current collector, wherein the cathode composition is rolled with a pressure roller during cell manufacture to smoothen the cathode and thereby permit one to use thinner electrolytes and reduce the impedance of the cell.
2. Description of the Prior Art
Solid state rechargeable electrochemical cells are the subject of intense investigation and development. They are described extensively in the patent literature. See, for example, U.S. Pat. Nos. 4,303,748 to Armand; 4,589,197 to North; 4,547,440 to Hooper, et al; and 4,228,226 to Christian. These cells are typically constructed of an alkali metal foil anode, typically lithium foil, an ionically conducting polymeric electrolyte, a composite cathode containing a finely divided transition metal oxide and an electrically-conductive filler, and a current collector. These cells present a viable option to older, more traditional, secondary cells because of their ability to generate a relatively high current per unit area and high storage capacity.
In solid state alkali metal anode laminar cells, internal cell impedance is the product of a number of factors. An internal impedance is associated with each element of the cell, namely the anode layer, the electrolyte layer, the cathode layer, and the current collector. Further, and as is particularly problematic, high impedance can occur at the interfaces between these layers and, in particular, at the electrolyte/cathode interface and at the cathode/current collector interface. Accordingly, to produce an efficient solid state electrochemical laminar cell, the impedance of each of these layers and interfaces must be minimized. The present invention seeks to provide as small an internal impedance as possible at the electrolyte/cathode interface and at the cathode/current collector interface.
Part of the problem in achieving a low cell impedance lies in the failure of the cathode material to form a good electric contact with the current collector. The increase in cell impedance associated with the lack of an optimal electrical contact makes it difficult to recharge the cell.
In theory, optimal performance occurs if the cathode material is in intimate contact with the cathode current collector, and wherein the cathode current collector has a high surface area to enable a uniform contact between the cathode material and the collector. Attempts have been made in the art to increase the adherence of the cathode material to the current collector. However, no such attempts have been made in the field of solid state alkali metal anode cells containing radiation curable cathodes.
In addition, increased cell impedance is associated with the electrolyte layer. The relationship between impedance and thickness of the electrolyte layer is directly proportional in that increased cell impedance occurs when utilizing a thicker electrolyte. Accordingly, it is desirable to minimize the coating thickness of the electrolyte layer.
U.S. Pat. No. 4,589,197 to North discloses a solid state lithium anode cell including a lithium anode, a lithium ion conducting polymeric electrolyte, a cathode based on an intercalation compound, a polymer, and an electrically conductive particle, and a current collector. To produce the cell, the cathode composition is cast in a sheet or film from a solution in a solvent, the solvent is removed, and the electrolyte and anode are assembled with the cathode and current collector. The cathode becomes porous as a result of the voids created during solvent removal. To increase the density of the cathode, the cathode film, after casting, may be rolled between the nip of two pressure rollers, or rolled by other means.
Accordingly, there exists a need in the art for a solid state alkali metal anode laminar cell which has a low overall impedance at the electrolyte/cathode and cathode/current collector interfaces, and is capable of discharging a high amount of current per unit area over a broad temperature range.